One of the most persistent myths is the
belief that Christopher Columbus sailed to the New World in order to prove that
the earth was round. In fact, learned people had known this for several
centuries before he was born. People had observed that the earth’s shadow
during a lunar eclipse is always circular. And they knew that the only object
that casts a circular shadow from every direction is a sphere.
And the natural next question to be
answered was how big that sphere is. Greek scientists proposed many answers
based on various conjectures. Remarkably, one of them actually figured it out.
In the 3rd century B.C.,
Eratosthenes, a native of Alexandria Egypt, realized something as simple as a
shadow could be used to solve this problem. He knew that on the summer
solstice, the sun was directly overhead at the southern Egyptian city of Syene
(modern day Aswan). In Alexandria, at the same time, the sun is at a 1/50th
declination from being directly overhead (which could be accurately measured
using shadows). Visit my author page at Lingua Sacra Publishing. You'll find everything from espionage thrillers, young adult adventure, and ways to learn new languages.
Taking just that information, Eratosthenes knew that he could calculate the
circumference of the earth as long as he knew how far apart
Alexandria and Syene are. Estimating that distance based merely on how long it
took him to go from one city to the other on a camel, he arrived at the figure
of 5000 stadia. There is some debate about what Eratosthenes’ stadia was equal
to. But if we assume that it was the Egyptian stadion, his calculations arrive
at 25,000 miles for the circumference of the earth. That is less than a hundred
miles off from what scientists today measure as the circumference (24,902).
When you consider that we also today know that the earth is not even a perfect
sphere (it bulges a bit at the equator), Eratosthenes breakthrough is
astounding.
The story of Eratosthenes shows us that
the solution to a problem can sometimes be a matter of approaching it from a
different angle. In this case, it let a man learn the true size of the world
for the first time, all without ever leaving Egypt.
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